14 This chapter will provide a basic introduction to system security concepts, some general good rules of thumb, and some advanced topics under DragonFly. A lot of the topics covered here can be applied to system and Internet security in general as well. The Internet is no longer a ***friendly*** place in which everyone wants to be your kind neighbor. Securing your system is imperative to protect your data, intellectual property, time, and much more from the hands of hackers and the like.

76 Security is a function that begins and ends with the system administrator. While all BSD UNIX® multi-user systems have some inherent security, the job of building and maintaining additional security mechanisms to keep those users ***honest*** is probably one of the single largest undertakings of the sysadmin. Machines are only as secure as you make them, and security concerns are ever competing with the human necessity for convenience. UNIX systems, in general, are capable of running a huge number of simultaneous processes and many of these processes operate as servers -- meaning that external entities can connect and talk to them. As yesterday's mini-computers and mainframes become today's desktops, and as computers become networked and internetworked, security becomes an even bigger issue.

80 Security is best implemented through a layered ***onion*** approach. In a nutshell, what you want to do is to create as many layers of security as are convenient and then carefully monitor the system for intrusions. You do not want to overbuild your security or you will interfere with the detection side, and detection is one of the single most important aspects of any security mechanism. For example, it makes little sense to set the `schg` flags (see [chflags(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#chflags&section1)) on every system binary because while this may temporarily protect the binaries, it prevents an attacker who has broken in from making an easily detectable change that may result in your security mechanisms not detecting the attacker at all.

84 System security also pertains to dealing with various forms of attack, including attacks that attempt to crash, or otherwise make a system unusable, but do not attempt to compromise the `root` account (***break root***). Security concerns can be split up into several categories:

100 A denial of service attack is an action that deprives the machine of needed resources. Typically, DoS attacks are brute-force mechanisms that attempt to crash or otherwise make a machine unusable by overwhelming its servers or network stack. Some DoS attacks try to take advantage of bugs in the networking stack to crash a machine with a single packet. The latter can only be fixed by applying a bug fix to the kernel. Attacks on servers can often be fixed by properly specifying options to limit the load the servers incur on the system under adverse conditions. Brute-force network attacks are harder to deal with. A spoofed-packet attack, for example, is nearly impossible to stop, short of cutting your system off from the Internet. It may not be able to take your machine down, but it can saturate your Internet connection.

104 A user account compromise is even more common than a DoS attack. Many sysadmins still run standard **telnetd** , **rlogind** , **rshd** , and **ftpd** servers on their machines. These servers, by default, do not operate over encrypted connections. The result is that if you have any moderate-sized user base, one or more of your users logging into your system from a remote location (which is the most common and convenient way to login to a system) will have his or her password sniffed. The attentive system admin will analyze his remote access logs looking for suspicious source addresses even for successful logins.

108 One must always assume that once an attacker has access to a user account, the attacker can break `root`. However, the reality is that in a well secured and maintained system, access to a user account does not necessarily give the attacker access to `root`. The distinction is important because without access to `root` the attacker cannot generally hide his tracks and may, at best, be able to do nothing more than mess with the user's files, or crash the machine. User account compromises are very common because users tend not to take the precautions that sysadmins take.

112 System administrators must keep in mind that there are potentially many ways to break `root` on a machine. The attacker may know the `root` password, the attacker may find a bug in a root-run server and be able to break `root` over a network connection to that server, or the attacker may know of a bug in a suid-root program that allows the attacker to break `root` once he has broken into a user's account. If an attacker has found a way to break `root` on a machine, the attacker may not have a need to install a backdoor. Many of the `root` holes found and closed to date involve a considerable amount of work by the attacker to cleanup after himself, so most attackers install backdoors. A backdoor provides the attacker with a way to easily regain `root` access to the system, but it also gives the smart system administrator a convenient way to detect the intrusion. Making it impossible for an attacker to install a backdoor may actually be detrimental to your security, because it will not close off the hole the attacker found to break in the first place.

156 **Command vs. Protocol:** Throughout this document, we will use **bold** text to refer to a command or application. This is used for instances such as ssh, since it is a protocol as well as command.

168 First off, do not bother securing staff accounts if you have not secured the `root` account. Most systems have a password assigned to the `root` account. The first thing you do is assume that the password is ***always*** compromised. This does not mean that you should remove the password. The password is almost always necessary for console access to the machine. What it does mean is that you should not make it possible to use the password outside of the console or possibly even with the [su(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#su&section1) command. For example, make sure that your pty's are specified as being insecure in the `/etc/ttys` file so that direct `root` logins via `telnet` or `rlogin` are disallowed. If using other login services such as **sshd** , make sure that direct `root` logins are disabled there as well. You can do this by editing your `/etc/ssh/sshd_config` file, and making sure that `PermitRootLogin` is set to `NO`. Consider every access method -- services such as FTP often fall through the cracks. Direct `root` logins should only be allowed via the system console.

172 Of course, as a sysadmin you have to be able to get to `root`, so we open up a few holes. But we make sure these holes require additional password verification to operate. One way to make `root` accessible is to add appropriate staff accounts to the `wheel` group (in `/etc/group`). The staff members placed in the `wheel` group are allowed to `su` to `root`. You should never give staff members native `wheel` access by putting them in the `wheel` group in their password entry. Staff accounts should be placed in a `staff` group, and then added to the `wheel` group via the `/etc/group` file. Only those staff members who actually need to have `root` access should be placed in the `wheel` group. It is also possible, when using an authentication method such as Kerberos, to use Kerberos' `.k5login` file in the `root` account to allow a [ksu(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ksu&section1) to `root` without having to place anyone at all in the `wheel` group. This may be the better solution since the `wheel` mechanism still allows an intruder to break `root` if the intruder has gotten hold of your password file and can break into a staff account. While having the `wheel` mechanism is better than having nothing at all, it is not necessarily the safest option.

176 An indirect way to secure staff accounts, and ultimately `root` access is to use an alternative login access method and do what is known as ***starring*** out the encrypted password for the staff accounts. Using the [vipw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#vipw&section8) command, one can replace each instance of an encrypted password with a single `*` character. This command will update the `/etc/master.passwd` file and user/password database to disable password-authenticated logins.

204 This change will prevent normal logins from occurring, since the encrypted password will never match `*`. With this done, staff members must use another mechanism to authenticate themselves such as [kerberos(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#kerberos&section1) or [ssh(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=ssh&section=1&manpath=OpenBSD+3.3) using a public/private key pair. When using something like Kerberos, one generally must secure the machines which run the Kerberos servers and your desktop workstation. When using a public/private key pair with ssh, one must generally secure the machine used to login ***from*** (typically one's workstation). An additional layer of protection can be added to the key pair by password protecting the key pair when creating it with [ssh-keygen(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=ssh-keygen&section=1). Being able to ***star*** out the passwords for staff accounts also guarantees that staff members can only login through secure access methods that you have set up. This forces all staff members to use secure, encrypted connections for all of their sessions, which closes an important hole used by many intruders: sniffing the network from an unrelated, less secure machine.

208 The more indirect security mechanisms also assume that you are logging in from a more restrictive server to a less restrictive server. For example, if your main box is running all sorts of servers, your workstation should not be running any. In order for your workstation to be reasonably secure you should run as few servers as possible, up to and including no servers at all, and you should run a password-protected screen blanker. Of course, given physical access to a workstation an attacker can break any sort of security you put on it. This is definitely a problem that you should consider, but you should also consider the fact that the vast majority of break-ins occur remotely, over a network, from people who do not have physical access to your workstation or servers.

212 Using something like Kerberos also gives you the ability to disable or change the password for a staff account in one place, and have it immediately affect all the machines on which the staff member may have an account. If a staff member's account gets compromised, the ability to instantly change his password on all machines should not be underrated. With discrete passwords, changing a password on N machines can be a mess. You can also impose re-passwording restrictions with Kerberos: not only can a Kerberos ticket be made to timeout after a while, but the Kerberos system can require that the user choose a new password after a certain period of time (say, once a month).

220 The prudent sysadmin only runs the servers he needs to, no more, no less. Be aware that third party servers are often the most bug-prone. For example, running an old version of **imapd** or **popper** is like giving a universal `root` ticket out to the entire world. Never run a server that you have not checked out carefully. Many servers do not need to be run as `root`. For example, the **ntalk** , **comsat** , and **finger** daemons can be run in special user ***sandboxes***. A sandbox is not perfect, unless you go through a large amount of trouble, but the onion approach to security still stands: If someone is able to break in through a server running in a sandbox, they still have to break out of the sandbox. The more layers the attacker must break through, the lower the likelihood of his success. Root holes have historically been found in virtually every server ever run as `root`, including basic system servers. If you are running a machine through which people only login via **sshd** and never login via **telnetd** or **rshd** or **rlogind** , then turn off those services!

224 DragonFly now defaults to running **ntalkd** , **comsat** , and **finger** in a sandbox. Another program which may be a candidate for running in a sandbox is [named(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#named&section8). `/etc/defaults/rc.conf` includes the arguments necessary to run **named** in a sandbox in a commented-out form. Depending on whether you are installing a new system or upgrading an existing system, the special user accounts used by these sandboxes may not be installed. The prudent sysadmin would research and implement sandboxes for servers whenever possible.

228 There are a number of other servers that typically do not run in sandboxes: **sendmail** , **popper** , **imapd** , **ftpd** , and others. There are alternatives to some of these, but installing them may require more work than you are willing to perform (the convenience factor strikes again). You may have to run these servers as `root` and rely on other mechanisms to detect break-ins that might occur through them.

232 The other big potential `root` holes in a system are the suid-root and sgid binaries installed on the system. Most of these binaries, such as **rlogin** , reside in `/bin`, `/sbin`, `/usr/bin`, or `/usr/sbin`. While nothing is 100% safe, the system-default suid and sgid binaries can be considered reasonably safe. Still, `root` holes are occasionally found in these binaries. A `root` hole was found in `Xlib` in 1998 that made **xterm** (which is typically suid) vulnerable. It is better to be safe than sorry and the prudent sysadmin will restrict suid binaries, that only staff should run, to a special group that only staff can access, and get rid of (`chmod 000`) any suid binaries that nobody uses. A server with no display generally does not need an **xterm** binary. Sgid binaries can be almost as dangerous. If an intruder can break an sgid-kmem binary, the intruder might be able to read `/dev/kmem` and thus read the encrypted password file, potentially compromising any passworded account. Alternatively an intruder who breaks group `kmem` can monitor keystrokes sent through pty's, including pty's used by users who login through secure methods. An intruder that breaks the `tty` group can write to almost any user's tty. If a user is running a terminal program or emulator with a keyboard-simulation feature, the intruder can potentially generate a data stream that causes the user's terminal to echo a command, which is then run as that user.

240 User accounts are usually the most difficult to secure. While you can impose Draconian access restrictions on your staff and ***star*** out their passwords, you may not be able to do so with any general user accounts you might have. If you do have sufficient control, then you may win out and be able to secure the user accounts properly. If not, you simply have to be more vigilant in your monitoring of those accounts. Use of ssh and Kerberos for user accounts is more problematic, due to the extra administration and technical support required, but still a very good solution compared to a crypted password file.

248 The only sure fire way is to `*` out as many passwords as you can and use ssh or Kerberos for access to those accounts. Even though the encrypted password file (`/etc/spwd.db`) can only be read by `root`, it may be possible for an intruder to obtain read access to that file even if the attacker cannot obtain root-write access.

260 If an attacker breaks `root` he can do just about anything, but there are certain conveniences. For example, most modern kernels have a packet sniffing device driver built in. Under DragonFly it is called the `bpf` device. An intruder will commonly attempt to run a packet sniffer on a compromised machine. You do not need to give the intruder the capability and most systems do not have the need for the `bpf` device compiled in.

264 But even if you turn off the `bpf` device, you still have `/dev/mem` and `/dev/kmem` to worry about. For that matter, the intruder can still write to raw disk devices. Also, there is another kernel feature called the module loader, [kldload(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#kldload&section8). An enterprising intruder can use a KLD module to install his own `bpf` device, or other sniffing device, on a running kernel. To avoid these problems you have to run the kernel at a higher secure level, at least securelevel 1. The securelevel can be set with a `sysctl` on the `kern.securelevel` variable. Once you have set the securelevel to 1, write access to raw devices will be denied and special `chflags` flags, such as `schg`, will be enforced. You must also ensure that the `schg` flag is set on critical startup binaries, directories, and script files -- everything that gets run up to the point where the securelevel is set. This might be overdoing it, and upgrading the system is much more difficult when you operate at a higher secure level. You may compromise and run the system at a higher secure level but not set the `schg` flag for every system file and directory under the sun. Another possibility is to simply mount `/` and `/usr` read-only. It should be noted that being too Draconian in what you attempt to protect may prevent the all-important detection of an intrusion.

272 When it comes right down to it, you can only protect your core system configuration and control files so much before the convenience factor rears its ugly head. For example, using `chflags` to set the `schg` bit on most of the files in `/` and `/usr` is probably counterproductive, because while it may protect the files, it also closes a detection window. The last layer of your security onion is perhaps the most important -- detection. The rest of your security is pretty much useless (or, worse, presents you with a false sense of safety) if you cannot detect potential incursions. Half the job of the onion is to slow down the attacker, rather than stop him, in order to give the detection side of the equation a chance to catch him in the act.

276 The best way to detect an incursion is to look for modified, missing, or unexpected files. The best way to look for modified files is from another (often centralized) limited-access system. Writing your security scripts on the extra-secure limited-access system makes them mostly invisible to potential attackers, and this is important. In order to take maximum advantage you generally have to give the limited-access box significant access to the other machines in the business, usually either by doing a read-only NFS export of the other machines to the limited-access box, or by setting up ssh key-pairs to allow the limited-access box to ssh to the other machines. Except for its network traffic, NFS is the least visible method -- allowing you to monitor the filesystems on each client box virtually undetected. If your limited-access server is connected to the client boxes through a switch, the NFS method is often the better choice. If your limited-access server is connected to the client boxes through a hub, or through several layers of routing, the NFS method may be too insecure (network-wise) and using ssh may be the better choice even with the audit-trail tracks that ssh lays.

280 Once you give a limited-access box, at least read access to the client systems it is supposed to monitor, you must write scripts to do the actual monitoring. Given an NFS mount, you can write scripts out of simple system utilities such as [find(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#find&section1) and [md5(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=md5&section=1). It is best to physically md5 the client-box files at least once a day, and to test control files such as those found in `/etc` and `/usr/local/etc` even more often. When mismatches are found, relative to the base md5 information the limited-access machine knows is valid, it should scream at a sysadmin to go check it out. A good security script will also check for inappropriate suid binaries and for new or deleted files on system partitions such as `/` and `/usr`.

284 When using ssh rather than NFS, writing the security script is much more difficult. You essentially have to `scp` the scripts to the client box in order to run them, making them visible, and for safety you also need to `scp` the binaries (such as find) that those scripts use. The **ssh** client on the client box may already be compromised. All in all, using ssh may be necessary when running over insecure links, but it is also a lot harder to deal with.

288 A good security script will also check for changes to user and staff members access configuration files: `.rhosts`, `.shosts`, `.ssh/authorized_keys` and so forth... files that might fall outside the purview of the `MD5` check.

292 If you have a huge amount of user disk space, it may take too long to run through every file on those partitions. In this case, setting mount flags to disallow suid binaries and devices on those partitions is a good idea. The `nodev` and `nosuid` options (see [mount(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#mount&section8)) are what you want to look into. You should probably scan them anyway, at least once a week, since the object of this layer is to detect a break-in whether or not the break-in is effective.

296 Process accounting (see [accton(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#accton&section8)) is a relatively low-overhead feature of the operating system which might help as a post-break-in evaluation mechanism. It is especially useful in tracking down how an intruder has actually broken into a system, assuming the file is still intact after the break-in occurs.

300 Finally, security scripts should process the log files, and the logs themselves should be generated in as secure a manner as possible -- remote syslog can be very useful. An intruder tries to cover his tracks, and log files are critical to the sysadmin trying to track down the time and method of the initial break-in. One way to keep a permanent record of the log files is to run the system console to a serial port and collect the information on a continuing basis through a secure machine monitoring the consoles.

308 A little paranoia never hurts. As a rule, a sysadmin can add any number of security features, as long as they do not affect convenience, and can add security features that ***do*** affect convenience with some added thought. Even more importantly, a security administrator should mix it up a bit -- if you use recommendations such as those given by this document verbatim, you give away your methodologies to the prospective attacker who also has access to this document.

316 This section covers Denial of Service attacks. A DoS attack is typically a packet attack. While there is not much you can do about modern spoofed packet attacks that saturate your network, you can generally limit the damage by ensuring that the attacks cannot take down your servers.

328 A common DoS attack is against a forking server that attempts to cause the server to eat processes, file descriptors, and memory, until the machine dies. **inetd** (see [inetd(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#inetd&section8)) has several options to limit this sort of attack. It should be noted that while it is possible to prevent a machine from going down, it is not generally possible to prevent a service from being disrupted by the attack. Read the **inetd** manual page carefully and pay specific attention to the `-c`, `-C`, and `-R` options. Note that spoofed-IP attacks will circumvent the `-C` option to **inetd** , so typically a combination of options must be used. Some standalone servers have self-fork-limitation parameters.

332 **Sendmail** has its `-OMaxDaemonChildren` option, which tends to work much better than trying to use sendmail's load limiting options due to the load lag. You should specify a `MaxDaemonChildren` parameter, when you start **sendmail** , high enough to handle your expected load, but not so high that the computer cannot handle that number of **sendmails** without falling on its face. It is also prudent to run sendmail in queued mode (`-ODeliveryMode=queued`) and to run the daemon (`sendmail -bd`) separate from the queue-runs (`sendmail -q15m`). If you still want real-time delivery you can run the queue at a much lower interval, such as `-q1m`, but be sure to specify a reasonable `MaxDaemonChildren` option for ***that*** sendmail to prevent cascade failures.

340 You should also be fairly careful with connect-back services such as **tcpwrapper** s reverse-identd, which can be attacked directly. You generally do not want to use the reverse-ident feature of **tcpwrappers** for this reason.

344 It is a very good idea to protect internal services from external access by firewalling them off at your border routers. The idea here is to prevent saturation attacks from outside your LAN, not so much to protect internal services from network-based `root` compromise. Always configure an exclusive firewall, i.e., firewall everything ***except*** ports A, B, C, D, and M-Z. This way you can firewall off all of your low ports except for certain specific services such as **named** (if you are primary for a zone), **ntalkd** , **sendmail** , and other Internet-accessible services. If you try to configure the firewall the other way -- as an inclusive or permissive firewall, there is a good chance that you will forget to ***close*** a couple of services, or that you will add a new internal service and forget to update the firewall. You can still open up the high-numbered port range on the firewall, to allow permissive-like operation, without compromising your low ports. Also take note that DragonFly allows you to control the range of port numbers used for dynamic binding, via the various `net.inet.ip.portrange` `sysctl`'s (`sysctl -a | fgrep portrange`), which can also ease the complexity of your firewall's configuration. For example, you might use a normal first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then block off everything under 4000 in your firewall (except for certain specific Internet-accessible ports, of course).

348 Another common DoS attack is called a springboard attack -- to attack a server in a manner that causes the server to generate responses which overloads the server, the local network, or some other machine. The most common attack of this nature is the ***ICMP ping broadcast attack***. The attacker spoofs ping packets sent to your LAN's broadcast address with the source IP address set to the actual machine they wish to attack. If your border routers are not configured to stomp on ping's to broadcast addresses, your LAN winds up generating sufficient responses to the spoofed source address to saturate the victim, especially when the attacker uses the same trick on several dozen broadcast addresses over several dozen different networks at once. Broadcast attacks of over a hundred and twenty megabits have been measured. A second common springboard attack is against the ICMP error reporting system. By constructing packets that generate ICMP error responses, an attacker can saturate a server's incoming network and cause the server to saturate its outgoing network with ICMP responses. This type of attack can also crash the server by running it out of mbuf's, especially if the server cannot drain the ICMP responses it generates fast enough. The DragonFly kernel has a new kernel compile option called `ICMP_BANDLIM` which limits the effectiveness of these sorts of attacks. The last major class of springboard attacks is related to certain internal **inetd** services such as the udp echo service. An attacker simply spoofs a UDP packet with the source address being server A's echo port, and the destination address being server B's echo port, where server A and B are both on your LAN. The two servers then bounce this one packet back and forth between each other. The attacker can overload both servers and their LANs simply by injecting a few packets in this manner. Similar problems exist with the internal **chargen** port. A competent sysadmin will turn off all of these inetd-internal test services.

352 Spoofed packet attacks may also be used to overload the kernel route cache. Refer to the `net.inet.ip.rtexpire`, `rtminexpire`, and `rtmaxcache` `sysctl` parameters. A spoofed packet attack that uses a random source IP will cause the kernel to generate a temporary cached route in the route table, viewable with `netstat -rna | fgrep W3`. These routes typically timeout in 1600 seconds or so. If the kernel detects that the cached route table has gotten too big it will dynamically reduce the `rtexpire` but will never decrease it to less than `rtminexpire`. There are two problems:

362 If your servers are connected to the Internet via a T3 or better, it may be prudent to manually override both `rtexpire` and `rtminexpire` via [sysctl(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#sysctl&section8). Never set either parameter to zero (unless you want to crash the machine). Setting both parameters to two seconds should be sufficient to protect the route table from attack.

370 There are a few issues with both Kerberos and ssh that need to be addressed if you intend to use them. Kerberos V is an excellent authentication protocol, but there are bugs in the kerberized **telnet** and **rlogin** applications that make them unsuitable for dealing with binary streams. Also, by default Kerberos does not encrypt a session unless you use the `-x` option. **ssh** encrypts everything by default.

374 ssh works quite well in every respect except that it forwards encryption keys by default. What this means is that if you have a secure workstation holding keys that give you access to the rest of the system, and you ssh to an insecure machine, your keys are usable. The actual keys themselves are not exposed, but ssh installs a forwarding port for the duration of your login, and if an attacker has broken `root` on the insecure machine he can utilize that port to use your keys to gain access to any other machine that your keys unlock.

378 We recommend that you use ssh in combination with Kerberos whenever possible for staff logins. **ssh** can be compiled with Kerberos support. This reduces your reliance on potentially exposable ssh keys while at the same time protecting passwords via Kerberos. ssh keys should only be used for automated tasks from secure machines (something that Kerberos is unsuited to do). We also recommend that you either turn off key-forwarding in the ssh configuration, or that you make use of the `from=IP/DOMAIN` option that ssh allows in its `authorized_keys` file to make the key only usable to entities logging in from specific machines.

393 Every user on a UNIX® system has a password associated with their account. It seems obvious that these passwords need to be known only to the user and the actual operating system. In order to keep these passwords secret, they are encrypted with what is known as a ***one-way hash***, that is, they can only be easily encrypted but not decrypted. In other words, what we told you a moment ago was obvious is not even true: the operating system itself does not ***really*** know the password. It only knows the ***encrypted*** form of the password. The only way to get the ***plain-text*** password is by a brute force search of the space of possible passwords.

397 Unfortunately the only secure way to encrypt passwords when UNIX came into being was based on DES, the Data Encryption Standard. This was not such a problem for users resident in the US, but since the source code for DES could not be exported outside the US, DragonFly had to find a way to both comply with US law and retain compatibility with all the other UNIX variants that still used DES.

401 The solution was to divide up the encryption libraries so that US users could install the DES libraries and use DES but international users still had an encryption method that could be exported abroad. This is how DragonFly came to use MD5 as its default encryption method. MD5 is believed to be more secure than DES, so installing DES is offered primarily for compatibility reasons.

413 It is pretty easy to identify which encryption method DragonFly is set up to use. Examining the encrypted passwords in the `/etc/master.passwd` file is one way. Passwords encrypted with the MD5 hash are longer than those encrypted with the DES hash and also begin with the characters `$1$`. Passwords starting with `$2a$` are encrypted with the Blowfish hash function. DES password strings do not have any particular identifying characteristics, but they are shorter than MD5 passwords, and are coded in a 64-character alphabet which does not include the `$` character, so a relatively short string which does not begin with a dollar sign is very likely a DES password.

417 The password format used for new passwords is controlled by the `passwd_format` login capability in `/etc/login.conf`, which takes values of `des`, `md5` or `blf`. See the [login.conf(5)](http://leaf.dragonflybsd.org/cgi/web-man?command#login.conf&amp;section5) manual page for more information about login capabilities.

428 S/Key is a one-time password scheme based on a one-way hash function. DragonFly uses the MD4 hash for compatibility but other systems have used MD5 and DES-MAC. S/Key ia part of the FreeBSD base system, and is also used on a growing number of other operating systems. S/Key is a registered trademark of Bell Communications Research, Inc.

432 There are three different sorts of passwords which we will discuss below. The first is your usual UNIX® style or Kerberos password; we will call this a ***UNIX password***. The second sort is the one-time password which is generated by the S/Key `key` program or the OPIE [opiekey(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#opiekey&section1) program and accepted by the `keyinit` or [opiepasswd(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=opiepasswd&section=1) programs and the login prompt; we will call this a ***one-time password***. The final sort of password is the secret password which you give to the `key`/`opiekey` programs (and sometimes the `keyinit`/`opiepasswd` programs) which it uses to generate one-time passwords; we will call it a ***secret password*** or just unqualified ***password***.

436 The secret password does not have anything to do with your UNIX password; they can be the same but this is not recommended. S/Key and OPIE secret passwords are not limited to eight characters like old UNIX passwords[(1)](#FTN.AEN8429), they can be as long as you like. Passwords of six or seven word long phrases are fairly common. For the most part, the S/Key or OPIE system operates completely independently of the UNIX password system.

440 Besides the password, there are two other pieces of data that are important to S/Key and OPIE. One is what is known as the ***seed*** or ***key***, consisting of two letters and five digits. The other is what is called the ***iteration count***, a number between 1 and 100. S/Key creates the one-time password by concatenating the seed and the secret password, then applying the MD4/MD5 hash as many times as specified by the iteration count and turning the result into six short English words. These six English words are your one-time password. The authentication system (primarily PAM) keeps track of the last one-time password used, and the user is authenticated if the hash of the user-provided password is equal to the previous password. Because a one-way hash is used it is impossible to generate future one-time passwords if a successfully used password is captured; the iteration count is decremented after each successful login to keep the user and the login program in sync. When the iteration count gets down to 1, S/Key and OPIE must be reinitialized.

444 There are three programs involved in each system which we will discuss below. The `key` and `opiekey` programs accept an iteration count, a seed, and a secret password, and generate a one-time password or a consecutive list of one-time passwords. The `keyinit` and `opiepasswd` programs are used to initialize S/Key and OPIE respectively, and to change passwords, iteration counts, or seeds; they take either a secret passphrase, or an iteration count, seed, and one-time password. The `keyinfo` and `opieinfo` programs examine the relevant credentials files (`/etc/skeykeys` or `/etc/opiekeys`) and print out the invoking user's current iteration count and seed.

448 There are four different sorts of operations we will cover. The first is using `keyinit` or `opiepasswd` over a secure connection to set up one-time-passwords for the first time, or to change your password or seed. The second operation is using `keyinit` or `opiepasswd` over an insecure connection, in conjunction with `key` or `opiekey` over a secure connection, to do the same. The third is using `key`/`opiekey` to log in over an insecure connection. The fourth is using `key` or `opiekey` to generate a number of keys which can be written down or printed out to carry with you when going to some location without secure connections to anywhere.

456 To initialize S/Key for the first time, change your password, or change your seed while logged in over a secure connection (e.g., on the console of a machine or via **ssh** ), use the `keyinit` command without any parameters while logged in as yourself:

516 At the Enter new secret pass phrase: or Enter secret password: prompts, you should enter a password or phrase. Remember, this is not the password that you will use to login with, this is used to generate your one-time login keys. The ***ID*** line gives the parameters of your particular instance: your login name, the iteration count, and seed. When logging in the system will remember these parameters and present them back to you so you do not have to remember them. The last line gives the particular one-time password which corresponds to those parameters and your secret password; if you were to re-login immediately, this one-time password is the one you would use.

524 To initialize or change your secret password over an insecure connection, you will need to already have a secure connection to some place where you can run `key` or `opiekey`; this might be in the form of a desk accessory on a Macintosh®, or a shell prompt on a machine you trust. You will also need to make up an iteration count (100 is probably a good value), and you may make up your own seed or use a randomly-generated one. Over on the insecure connection (to the machine you are initializing), use the `keyinit -s` command:

588 To accept the default seed (which the `keyinit` program confusingly calls a `key`), press **Return** . Then before entering an access password, move over to your secure connection or S/Key desk accessory, and give it the same parameters:

694 As a side note, the S/Key and OPIE prompts have a useful feature (not shown here): if you press **Return** at the password prompt, the prompter will turn echo on, so you can see what you are typing. This can be extremely useful if you are attempting to type in a password by hand, such as from a printout.

698 At this point you need to generate your one-time password to answer this login prompt. This must be done on a trusted system that you can run `key` or `opiekey` on. (There are versions of these for DOS, Windows® and Mac OS® as well.) They need both the iteration count and the seed as command line options. You can cut-and-paste these right from the login prompt on the machine that you are logging in to.

766 Sometimes you have to go places where you do not have access to a trusted machine or secure connection. In this case, it is possible to use the `key` and `opiekey` commands to generate a number of one-time passwords beforehand to be printed out and taken with you. For example:

820 The `-n 5` requests five keys in sequence, the `30` specifies what the last iteration number should be. Note that these are printed out in ***reverse*** order of eventual use. If you are really paranoid, you might want to write the results down by hand; otherwise you can cut-and-paste into `lpr`. Note that each line shows both the iteration count and the one-time password; you may still find it handy to scratch off passwords as you use them.

828 S/Key can place restrictions on the use of UNIX passwords based on the host name, user name, terminal port, or IP address of a login session. These restrictions can be found in the configuration file `/etc/skey.access`. The [skey.access(5)](http://leaf.dragonflybsd.org/cgi/web-man?command#skey.access&section5) manual page has more information on the complete format of the file and also details some security cautions to be aware of before depending on this file for security.

832 If there is no `/etc/skey.access` file (this is the default), then all users will be allowed to use UNIX passwords. If the file exists, however, then all users will be required to use S/Key unless explicitly permitted to do otherwise by configuration statements in the `skey.access` file. In all cases, UNIX passwords are permitted on the console.

852 The first line (`permit internet`) allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use UNIX passwords. This should not be considered a security mechanism, but rather, a means to remind authorized users that they are using an insecure network and need to use S/Key for authentication.

856 The second line (`permit user`) allows the specified username, in this case `fnord`, to use UNIX passwords at any time. Generally speaking, this should only be used for people who are either unable to use the `key` program, like those with dumb terminals, or those who are uneducable.

916 **Kerberos** is a network add-on system/protocol that allows users to authenticate themselves through the services of a secure server. Services such as remote login, remote copy, secure inter-system file copying and other high-risk tasks are made considerably safer and more controllable.

920 **Kerberos** can be described as an identity-verifying proxy system. It can also be described as a trusted third-party authentication system. **Kerberos** provides only one function -- the secure authentication of users on the network. It does not provide authorization functions (what users are allowed to do) or auditing functions (what those users did). After a client and server have used **Kerberos** to prove their identity, they can also encrypt all of their communications to assure privacy and data integrity as they go about their business.

952 **Kerberos** was created by MIT as a solution to network security problems. The **Kerberos** protocol uses strong cryptography so that a client can prove its identity to a server (and vice versa) across an insecure network connection.

956 **Kerberos** is both the name of a network authentication protocol and an adjective to describe programs that implement the program ( **Kerberos** telnet, for example). The current version of the protocol is version 5, described in RFC 1510.

960 Several free implementations of this protocol are available, covering a wide range of operating systems. The Massachusetts Institute of Technology (MIT), where **Kerberos** was originally developed, continues to develop their **Kerberos** package. It is commonly used in the US as a cryptography product, as such it has historically been affected by US export regulations. The MIT **Kerberos** is available as a port ([`security/krb5`](http://pkgsrc.se/security/krb5)). Heimdal **Kerberos** is another version 5 implementation, and was explicitly developed outside of the US to avoid export regulations (and is thus often included in non-commercial UNIX® variants). The Heimdal **Kerberos** distribution is available as a port ([`security/heimdal`](http://pkgsrc.se/security/heimdal)), and a minimal installation of it is included in the base DragonFly install.

972 The Key Distribution Center (KDC) is the centralized authentication service that **Kerberos** provides -- it is the computer that issues **Kerberos** tickets. The KDC is considered ***trusted*** by all other computers in the **Kerberos** realm, and thus has heightened security concerns.

1022 Note that this `/etc/krb5.conf` file implies that your KDC will have the fully-qualified hostname of `kerberos.example.org`. You will need to add a CNAME (alias) entry to your zone file to accomplish this if your KDC has a different hostname.

1060 Next we will create the **Kerberos** database. This database contains the keys of all principals encrypted with a master password. You are not required to remember this password, it will be stored in a file (`/var/heimdal/m-key`). To create the master key, run `kstash` and enter a password.

1064 Once the master key has been created, you can initialize the database using the `kadmin` program with the `-l` option (standing for ***local***). This option instructs `kadmin` to modify the database files directly rather than going through the `kadmind` network service. This handles the chicken-and-egg problem of trying to connect to the database before it is created. Once you have the `kadmin` prompt, use the `init` command to create your realms initial database.

1068 Lastly, while still in `kadmin`, create your first principal using the `add` command. Stick to the defaults options for the principal for now, you can always change them later with the `modify` command. Note that you can use the `?` command at any prompt to see the available options.

1108 Now it is time to start up the KDC services. Run `/etc/rc.d/kerberos start` and `/etc/rc.d/kadmind start` to bring up the services. Note that you won't have any kerberized daemons running at this point but you should be able to confirm that the KDC is functioning by obtaining and listing a ticket for the principal (user) that you just created from the command-line of the KDC itself:

1140 First, we need a copy of the **Kerberos** configuration file, `/etc/krb5.conf`. To do so, simply copy it over to the client computer from the KDC in a secure fashion (using network utilities, such as [scp(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#scp&section1&manpath=OpenBSD+3.3), or physically via a floppy disk).

1144 Next you need a `/etc/krb5.keytab` file. This is the major difference between a server providing **Kerberos** enabled daemons and a workstation -- the server must have a `keytab` file. This file contains the servers host key, which allows it and the KDC to verify each others identity. It must be transmitted to the server in a secure fashion, as the security of the server can be broken if the key is made public. This explicitly means that transferring it via a clear text channel, such as FTP, is a very bad idea.

1148 Typically, you transfer to the `keytab` to the server using the `kadmin` program. This is handy because you also need to create the host principal (the KDC end of the `krb5.keytab`) using `kadmin`.

1152 Note that you must have already obtained a ticket and that this ticket must be allowed to use the `kadmin` interface in the `kadmind.acl`. See the section titled ***Remote administration*** in the Heimdal info pages (`info heimdal`) for details on designing access control lists. If you do not want to enable remote `kadmin` access, you can simply securely connect to the KDC (via local console, [ssh(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh&section1&manpath=OpenBSD+3.3) or **Kerberos** [telnet(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=telnet&section=1)) and perform administration locally using `kadmin -l`.

1156 After installing the `/etc/krb5.conf` file, you can use `kadmin` from the **Kerberos** server. The `add --random-key` command will let you add the servers host principal, and the `ext` command will allow you to extract the servers host principal to its own keytab. For example:

1184 If you do not have `kadmind` running on the KDC (possibly for security reasons) and thus do not have access to `kadmin` remotely, you can add the host principal (`host/myserver.EXAMPLE.ORG`) directly on the KDC and then extract it to a temporary file (to avoid over-writing the `/etc/krb5.keytab` on the KDC) using something like this:

1204 At this point your server can communicate with the KDC (due to its `krb5.conf` file) and it can prove its own identity (due to the `krb5.keytab` file). It is now ready for you to enable some **Kerberos** services. For this example we will enable the `telnet` service by putting a line like this into your `/etc/inetd.conf` and then restarting the [inetd(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#inetd&section8) service with `/etc/rc.d/inetd restart`:

1216 The critical bit is that the `-a` (for authentication) type is set to user. Consult the [telnetd(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#telnetd&section8) manual page for more details.

1224 Setting up a client computer is almost trivially easy. As far as **Kerberos** configuration goes, you only need the **Kerberos** configuration file, located at `/etc/krb5.conf`. Simply securely copy it over to the client computer from the KDC.

1228 Test your client computer by attempting to use `kinit`, `klist`, and `kdestroy` from the client to obtain, show, and then delete a ticket for the principal you created above. You should also be able to use **Kerberos** applications to connect to **Kerberos** enabled servers, though if that does not work and obtaining a ticket does the problem is likely with the server and not with the client or the KDC.

1232 When testing an application like `telnet`, try using a packet sniffer (such as [tcpdump(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#tcpdump&section1)) to confirm that your password is not sent in the clear. Try using `telnet` with the `-x` option, which encrypts the entire data stream (similar to `ssh`).

1236 The core **Kerberos** client applications (traditionally named `kinit`, `klist`, `kdestroy`, and `kpasswd`) are installed in the base DragonFly install. Note that DragonFly versions prior to 5.0 renamed them to `k5init`, `k5list`, `k5destroy`, `k5passwd`, and `k5stash` (though it is typically only used once).

1240 Various non-core **Kerberos** client applications are also installed by default. This is where the ***minimal*** nature of the base Heimdal installation is felt: `telnet` is the only **Kerberos** enabled service.

1244 The Heimdal port adds some of the missing client applications: **Kerberos** enabled versions of `ftp`, `rsh`, `rcp`, `rlogin`, and a few other less common programs. The MIT port also contains a full suite of **Kerberos** client applications.

1252 Users within a realm typically have their **Kerberos** principal (such as `tillman@EXAMPLE.ORG`) mapped to a local user account (such as a local account named `tillman`). Client applications such as `telnet` usually do not require a user name or a principal.

1256 Occasionally, however, you want to grant access to a local user account to someone who does not have a matching **Kerberos** principal. For example, `tillman@EXAMPLE.ORG` may need access to the local user account `webdevelopers`. Other principals may also need access to that local account.

1260 The `.k5login` and `.k5users` files, placed in a users home directory, can be used similar to a powerful combination of `.hosts` and `.rhosts`, solving this problem. For example, if a `.k5login` with the following contents:

1296 * If you change your hostname, you also need to change your `host/` principal and update your keytab. This also applies to special keytab entries like the `www/` principal used for Apache's [`www/mod_auth_kerb`](http://pkgsrc.se/www/mod_auth_kerb).

1299 * All hosts in your realm must be resolvable (both forwards and reverse) in DNS (or `/etc/hosts` as a minimum). CNAMEs will work, but the A and PTR records must be correct and in place. The error message isn't very intuitive: ***`Kerberos5 refuses authentication because Read req failed: Key table entry not found`***.

1302 * Some operating systems that may being acting as clients to your KDC do not set the permissions for `ksu` to be setuid `root`. This means that `ksu` does not work, which is a good security idea but annoying. This is not a KDC error.

1305 * With MIT **Kerberos** , if you want to allow a principal to have a ticket life longer than the default ten hours, you must use `modify_principal` in `kadmin` to change the maxlife of both the principal in question and the `krbtgt` principal. Then the principal can use the `-l` option with `kinit` to request a ticket with a longer lifetime.

1308 * **Note:** If you run a packet sniffer on your KDC to add in troubleshooting and then run `kinit` from a workstation, you will notice that your TGT is sent immediately upon running `kinit` -- even before you type your password! The explanation is that the **Kerberos** server freely transmits a TGT (Ticket Granting Ticket) to any unauthorized request; however, every TGT is encrypted in a key derived from the user's password. Therefore, when a user types their password it is not being sent to the KDC, it is being used to decrypt the TGT that `kinit` already obtained. If the decryption process results in a valid ticket with a valid time stamp, the user has valid **Kerberos** credentials. These credentials include a session key for establishing secure communications with the **Kerberos** server in the future, as well as the actual ticket-granting ticket, which is actually encrypted with the **Kerberos** server's own key. This second layer of encryption is unknown to the user, but it is what allows the **Kerberos** server to verify the authenticity of each TGT.

1314 * If you want to use long ticket lifetimes (a week, for example) and you are using **OpenSSH** to connect to the machine where your ticket is stored, make sure that **Kerberos** `TicketCleanup` is set to `no` in your `sshd_config` or else your tickets will be deleted when you log out.

1317 * Remember that host principals can have a longer ticket lifetime as well. If your user principal has a lifetime of a week but the host you are connecting to has a lifetime of nine hours, you will have an expired host principal in your cache and the ticket cache will not work as expected.

1320 * When setting up a `krb5.dict` file to prevent specific bad passwords from being used (the manual page for `kadmind` covers this briefly), remember that it only applies to principals that have a password policy assigned to them. The `krb5.dict` files format is simple: one string per line. Creating a symbolic link to `/usr/share/dict/words` might be useful.

1328 The major difference between the MIT and Heimdal installs relates to the `kadmin` program which has a different (but equivalent) set of commands and uses a different protocol. This has a large implications if your KDC is MIT as you will not be able to use the Heimdal `kadmin` program to administer your KDC remotely (or vice versa, for that matter).

1332 The client applications may also take slightly different command line options to accomplish the same tasks. Following the instructions on the MIT **Kerberos** web site (http://web.mit.edu/Kerberos/www/) is recommended. Be careful of path issues: the MIT port installs into `/usr/local/` by default, and the ***normal*** system applications may be run instead of MIT if your `PATH` environment variable lists the system directories first.

1336 **Note:** With the MIT [`security/krb5`](http://pkgsrc.se/security/krb5) port that is provided by DragonFly, be sure to read the `/usr/local/share/doc/krb5/README.FreeBSD` file installed by the port if you want to understand why logins via `telnetd` and `klogind` behave somewhat oddly. Most importantly, correcting the ***incorrect permissions on cache file*** behavior requires that the `login.krb5` binary be used for authentication so that it can properly change ownership for the forwarded credentials.

1348 Every service enabled on the network must be modified to work with **Kerberos** (or be otherwise secured against network attacks) or else the users credentials could be stolen and re-used. An example of this would be **Kerberos** enabling all remote shells (via `rsh` and `telnet`, for example) but not converting the POP3 mail server which sends passwords in plaintext.

1356 In a multi-user environment, **Kerberos** is less secure. This is because it stores the tickets in the `/tmp` directory, which is readable by all users. If a user is sharing a computer with several other people simultaneously (i.e. multi-user), it is possible that the user's tickets can be stolen (copied) by another user.

1360 This can be overcome with the `-c` filename command-line option or (preferably) the `KRB5CCNAME` environment variable, but this is rarely done. In principal, storing the ticket in the users home directory and using simple file permissions can mitigate this problem.

1368 By design, the KDC must be as secure as the master password database is contained on it. The KDC should have absolutely no other services running on it and should be physically secured. The danger is high because **Kerberos** stores all passwords encrypted with the same key (the ***master*** key), which in turn is stored as a file on the KDC.

1372 As a side note, a compromised master key is not quite as bad as one might normally fear. The master key is only used to encrypt the **Kerberos** database and as a seed for the random number generator. As long as access to your KDC is secure, an attacker cannot do much with the master key.

1376 Additionally, if the KDC is unavailable (perhaps due to a denial of service attack or network problems) the network services are unusable as authentication can not be performed, a recipe for a denial-of-service attack. This can alleviated with multiple KDCs (a single master and one or more slaves) and with careful implementation of secondary or fall-back authentication (PAM is excellent for this).

1384 **Kerberos** allows users, hosts and services to authenticate between themselves. It does not have a mechanism to authenticate the KDC to the users, hosts or services. This means that a trojanned `kinit` (for example) could record all user names and passwords. Something like [`security/tripwire`](http://pkgsrc.se/security/tripwire) or other file system integrity checking tools can alleviate this.

1431 Firewalls are an area of increasing interest for people who are connected to the Internet, and are even finding applications on private networks to provide enhanced security. This section will hopefully explain what firewalls are, how to use them, and how to use the facilities provided in the DragonFly kernel to implement them.

1435 **Note:** People often think that having a firewall between your internal network and the ***Big Bad Internet*** will solve all your security problems. It may help, but a poorly set up firewall system is more of a security risk than not having one at all. A firewall can add another layer of security to your systems, but it cannot stop a really determined cracker from penetrating your internal network. If you let internal security lapse because you believe your firewall to be impenetrable, you have just made the crackers job that much easier.

1443 There are currently two distinct types of firewalls in common use on the Internet today. The first type is more properly called a ***packet filtering router***. This type of firewall utilizes a multi-homed machine and a set of rules to determine whether to forward or block individual packets. A multi-homed machine is simply a device with multiple network interfaces. The second type, known as a ***proxy server***, relies on daemons to provide authentication and to forward packets, possibly on a multi-homed machine which has kernel packet forwarding disabled.

1447 Sometimes sites combine the two types of firewalls, so that only a certain machine (known as a ***bastion host***) is allowed to send packets through a packet filtering router onto an internal network. Proxy services are run on the bastion host, which are generally more secure than normal authentication mechanisms.

1451 DragonFly comes with a kernel packet filter (known as IPFW), which is what the rest of this section will concentrate on. Proxy servers can be built on DragonFly from third party software, but there is such a variety of proxy servers available that it would be impossible to cover them in this section.

1459 A router is a machine which forwards packets between two or more networks. A packet filtering router is programmed to compare each packet to a list of rules before deciding if it should be forwarded or not. Most modern IP routing software includes packet filtering functionality that defaults to forwarding all packets. To enable the filters, you need to define a set of rules.

1463 To decide whether a packet should be passed on, the firewall looks through its set of rules for a rule which matches the contents of the packet's headers. Once a match is found, the rule action is obeyed. The rule action could be to drop the packet, to forward the packet, or even to send an ICMP message back to the originator. Only the first match counts, as the rules are searched in order. Hence, the list of rules can be referred to as a ***rule chain***.

1467 The packet-matching criteria varies depending on the software used, but typically you can specify rules which depend on the source IP address of the packet, the destination IP address, the source port number, the destination port number (for protocols which support ports), or even the packet type (UDP, TCP, ICMP, etc).

1475 Proxy servers are machines which have had the normal system daemons ( **telnetd** , **ftpd** , etc) replaced with special servers. These servers are called ***proxy servers***, as they normally only allow onward connections to be made. This enables you to run (for example) a proxy **telnet** server on your firewall host, and people can **telnet** in to your firewall from the outside, go through some authentication mechanism, and then gain access to the internal network (alternatively, proxy servers can be used for signals coming from the internal network and heading out).

1479 Proxy servers are normally more secure than normal servers, and often have a wider variety of authentication mechanisms available, including ***one-shot*** password systems so that even if someone manages to discover what password you used, they will not be able to use it to gain access to your systems as the password expires immediately after the first use. As they do not actually give users access to the host machine, it becomes a lot more difficult for someone to install backdoors around your security system.

1483 Proxy servers often have ways of restricting access further, so that only certain hosts can gain access to the servers. Most will also allow the administrator to specify which users can talk to which destination machines. Again, what facilities are available depends largely on what proxy software you choose.

1491 IPFW, the software supplied with DragonFly, is a packet filtering and accounting system which resides in the kernel, and has a user-land control utility, [ipfw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipfw&section8). Together, they allow you to define and query the rules used by the kernel in its routing decisions.

1495 There are two related parts to IPFW. The firewall section performs packet filtering. There is also an IP accounting section which tracks usage of the router, based on rules similar to those used in the firewall section. This allows the administrator to monitor how much traffic the router is getting from a certain machine, or how much WWW traffic it is forwarding, for example.

1499 As a result of the way that IPFW is designed, you can use IPFW on non-router machines to perform packet filtering on incoming and outgoing connections. This is a special case of the more general use of IPFW, and the same commands and techniques should be used in this situation.

1507 As the main part of the IPFW system lives in the kernel, you will need to add one or more options to your kernel configuration file, depending on what facilities you want, and recompile your kernel. See "Reconfiguring your Kernel" ([kernelconfig.html Chapter 9]) for more details on how to recompile your kernel.

1511 **Warning:** IPFW defaults to a policy of `deny ip from any to any`. If you do not add other rules during startup to allow access, ***you will lock yourself out*** of the server upon rebooting into a firewall-enabled kernel. We suggest that you set `firewall_type=open` in your `/etc/rc.conf` file when first enabling this feature, then refining the firewall rules in `/etc/rc.firewall` after you have tested that the new kernel feature works properly. To be on the safe side, you may wish to consider performing the initial firewall configuration from the local console rather than via **ssh** . Another option is to build a kernel using both the `IPFIREWALL` and `IPFIREWALL_DEFAULT_TO_ACCEPT` options. This will change the default rule of IPFW to `allow ip from any to any` and avoid the possibility of a lockout.

1519 `options IPFIREWALL`:: Compiles into the kernel the code for packet filtering.`options IPFIREWALL_VERBOSE`:: Enables code to allow logging of packets through [syslogd(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#syslogd&section8). Without this option, even if you specify that packets should be logged in the filter rules, nothing will happen.`options IPFIREWALL_VERBOSE_LIMIT=10`:: Limits the number of packets logged through [syslogd(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=syslogd&section=8) on a per entry basis. You may wish to use this option in hostile environments in which you want to log firewall activity, but do not want to be open to a denial of service attack via syslog flooding.

1521 When a chain entry reaches the packet limit specified, logging is turned off for that particular entry. To resume logging, you will need to reset the associated counter using the [ipfw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipfw&section8) utility:

1529 Where 4500 is the chain entry you wish to continue logging.`options IPFIREWALL_DEFAULT_TO_ACCEPT`:: This changes the default rule action from ***deny*** to ***allow***. This avoids the possibility of locking yourself out if you happen to boot a kernel with `IPFIREWALL` support but have not configured your firewall yet. It is also very useful if you often use [ipfw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipfw&section8) as a filter for specific problems as they arise. Use with care though, as this opens up the firewall and changes the way it works.

1537 The configuration of the IPFW software is done through the [ipfw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipfw&section8) utility. The syntax for this command looks quite complicated, but it is relatively simple once you understand its structure.

1541 There are currently four different command categories used by the utility: addition/deletion, listing, flushing, and clearing. Addition/deletion is used to build the rules that control how packets are accepted, rejected, and logged. Listing is used to examine the contents of your rule set (otherwise known as the chain) and packet counters (accounting). Flushing is used to remove all entries from the chain. Clearing is used to zero out one or more accounting entries.

1577 If an ***index*** value is supplied, it is used to place the entry at a specific point in the chain. Otherwise, the entry is placed at the end of the chain at an index 100 greater than the last chain entry (this does not include the default policy, rule 65535, deny).

1589 reject:: Drop the packet, and send an ICMP host or port unreachable (as appropriate) packet to the source.allow:: Pass the packet on as normal. (aliases: `pass`, `permit`, and `accept`)deny:: Drop the packet. The source is not notified via an ICMP message (thus it appears that the packet never arrived at the destination).count:: Update packet counters but do not allow/deny the packet based on this rule. The search continues with the next chain entry.

1617 The `via` is optional and may specify the IP address or domain name of a local IP interface, or an interface name (e.g. `ed0`) to match only packets coming through this interface. Interface unit numbers can be specified with an optional wildcard. For example, `ppp*` would match all kernel PPP interfaces.

1657 A valid hostname may be specified in place of the IP address. ' **mask-bits** ' is a decimal number representing how many bits in the address mask should be set. e.g. specifying `192.216.222.1/24` will create a mask which will allow any address in a class C subnet (in this case, `192.216.222`) to be matched. ' **mask-pattern** ' is an IP address which will be logically AND'ed with the address given. The keyword `any` may be used to specify ***any IP address***.

1685 frag:: Matches if the packet is not the first fragment of the datagram.in:: Matches if the packet is on the way in.out:: Matches if the packet is on the way out.ipoptions `***spec***`:: Matches if the IP header contains the comma separated list of options specified in `***spec***`. The supported IP options are: `ssrr` (strict source route), `lsrr` (loose source route), `rr` (record packet route), and `ts` (time stamp). The absence of a particular option may be specified with a leading `!`.established:: Matches if the packet is part of an already established TCP connection (i.e. it has the RST or ACK bits set). You can optimize the performance of the firewall by placing ***established*** rules early in the chain.setup:: Matches if the packet is an attempt to establish a TCP connection (the SYN bit is set but the ACK bit is not).tcpflags `***flags***`:: Matches if the TCP header contains the comma separated list of `***flags***`. The supported flags are `fin`, `syn`, `rst`, `psh`, `ack`, and `urg`. The absence of a particular flag may be indicated by a leading `!`.icmptypes `***types***`:: Matches if the ICMP type is present in the list `***types***`. The list may be specified as any combination of ranges and/or individual types separated by commas. Commonly used ICMP types are: `0` echo reply (ping reply), `3` destination unreachable, `5` redirect, `8` echo request (ping request), and `11` time exceeded (used to indicate TTL expiration as with [traceroute(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#traceroute&section8)).

1705 -a:: While listing, show counter values. This option is the only way to see accounting counters.-c:: List rules in compact form.-d:: Show dynamic rules in addition to static rules.-e:: If `-d` was specified, also show expired dynamic rules.-t:: Display the last match times for each chain entry. The time listing is incompatible with the input syntax used by the [ipfw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipfw&section8) utility.-N:: Attempt to resolve given addresses and service names.-S:: Show the set each rule belongs to. If this flag is not specified, disabled rules will not be listed.

1721 This causes all entries in the firewall chain to be removed except the fixed default policy enforced by the kernel (index 65535). Use caution when flushing rules; the default deny policy will leave your system cut off from the network until allow entries are added to the chain.

1822 **Note:** The following suggestions are just that: suggestions. The requirements of each firewall are different and we cannot tell you how to build a firewall to meet your particular requirements.

1826 When initially setting up your firewall, unless you have a test bench setup where you can configure your firewall host in a controlled environment, it is strongly recommend you use the logging version of the commands and enable logging in the kernel. This will allow you to quickly identify problem areas and cure them without too much disruption. Even after the initial setup phase is complete, I recommend using the logging for `deny' as it allows tracing of possible attacks and also modification of the firewall rules if your requirements alter.

1830 **Note:** If you use the logging versions of the `accept` command, be aware that it can generate ***large*** amounts of log data. One log entry will be generated for every packet that passes through the firewall, so large FTP/http transfers, etc, will really slow the system down. It also increases the latencies on those packets as it requires more work to be done by the kernel before the packet can be passed on. **syslogd** will also start using up a lot more processor time as it logs all the extra data to disk, and it could quite easily fill the partition `/var/log` is located on.

1834 You should enable your firewall from `/etc/rc.conf.local` or `/etc/rc.conf`. The associated manual page explains which knobs to fiddle and lists some preset firewall configurations. If you do not use a preset configuration, `ipfw list` will output the current ruleset into a file that you can pass to `rc.conf`. If you do not use `/etc/rc.conf.local` or `/etc/rc.conf` to enable your firewall, it is important to make sure your firewall is enabled before any IP interfaces are configured.

1838 The next problem is what your firewall should actually ***do***! This is largely dependent on what access to your network you want to allow from the outside, and how much access to the outside world you want to allow from the inside. Some general rules are:

1846 * Block ***all*** incoming UDP traffic. There are very few useful services that travel over UDP, and what useful traffic there is, is normally a security threat (e.g. Suns RPC and NFS protocols). This has its disadvantages also, since UDP is a connectionless protocol, denying incoming UDP traffic also blocks the replies to outgoing UDP traffic. This can cause a problem for people (on the inside) using external archie (prospero) servers. If you want to allow access to archie, you will have to allow packets coming from ports 191 and 1525 to any internal UDP port through the firewall. **ntp** is another service you may consider allowing through, which comes from port 123.

1849 * Block traffic to port 6000 from the outside. Port 6000 is the port used for access to X11 servers, and can be a security threat (especially if people are in the habit of doing `xhost +` on their workstations). X11 can actually use a range of ports starting at 6000, the upper limit being how many X displays you can run on the machine. The upper limit as defined by RFC 1700 (Assigned Numbers) is 6063.

1860 As stated above, these are only ***guidelines***. You will have to decide what filter rules you want to use on your firewall yourself. We cannot accept ANY responsibility if someone breaks into your network, even if you follow the advice given above.

1868 Many people want to know how much overhead IPFW adds to a system. The answer to this depends mostly on your rule set and processor speed. For most applications dealing with Ethernet and small rule sets, the answer is ***negligible***. For those of you that need actual measurements to satisfy your curiosity, read on.

1872 The following measurements were made using FreeBSD 2.2.5-STABLE on a 486-66. (While IPFW has changed slightly in later releases of DragonFly, it still performs with similar speed.) IPFW was modified to measure the time spent within the `ip_fw_chk` routine, displaying the results to the console every 1000 packets.

1888 This demonstrates a worst case scenario by causing most of IPFW's packet check routine to be executed before finally deciding that the packet does not match the rule (by virtue of the port number). Following the 999th iteration of this rule was an `allow ip from any to any`.

1908 The per-packet processing overhead in the former case was approximately 2.703 ms/packet, or roughly 2.7 microseconds per rule. Thus the theoretical packet processing limit with these rules is around 370 packets per second. Assuming 10 Mbps Ethernet and a ~1500 byte packet size, we would only be able to achieve 55.5% bandwidth utilization.

1912 For the latter case each packet was processed in approximately 1.172 ms, or roughly 1.2 microseconds per rule. The theoretical packet processing limit here would be about 853 packets per second, which could consume 10 Mbps Ethernet bandwidth.

1916 The excessive number of rules tested and the nature of those rules do not provide a real-world scenario -- they were used only to generate the timing information presented here. Here are a few things to keep in mind when building an efficient rule set:

1924 * Place heavily triggered rules earlier in the rule set than those rarely used (***without changing the permissiveness of the firewall***, of course). You can see which rules are used most often by examining the packet counting statistics with `ipfw -a l`.

1949 However, one of the algorithms (specifically IDEA) included in OpenSSL is protected by patents in the USA and elsewhere, and is not available for unrestricted use. IDEA is included in the OpenSSL sources in DragonFly, but it is not built by default. If you wish to use it, and you comply with the license terms, enable the `MAKE_IDEA` switch in `/etc/make.conf` and rebuild your sources using `make world`.

1980 This section will guide you through the process of setting up IPsec, and to use it in an environment which consists of DragonFly and **Microsoft® Windows® 2000/XP** machines, to make them communicate securely. In order to set up IPsec, it is necessary that you are familiar with the concepts of building a custom kernel (see [kernelconfig.html Chapter 9]).

1984 ***IPsec*** is a protocol which sits on top of the Internet Protocol (IP) layer. It allows two or more hosts to communicate in a secure manner (hence the name). The DragonFly IPsec ***network stack*** is based on the [KAME](http://www.kame.net/) implementation, which has support for both protocol families, IPv4 and IPv6.

1996 * ***Authentication Header (AH)***, protects the IP packet header from third party interference and spoofing, by computing a cryptographic checksum and hashing the IP packet header fields with a secure hashing function. This is then followed by an additional header that contains the hash, to allow the information in the packet to be authenticated.

2004 IPsec can either be used to directly encrypt the traffic between two hosts (known as ***Transport Mode***); or to build ***virtual tunnels*** between two subnets, which could be used for secure communication between two corporate networks (known as ***Tunnel Mode***). The latter is more commonly known as a ***Virtual Private Network (VPN)***. The [ipsec(4)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipsec&section4) manual page should be consulted for detailed information on the IPsec subsystem in DragonFly.

2042 There's no standard for what constitutes a VPN. VPNs can be implemented using a number of different technologies, each of which have their own strengths and weaknesses. This article presents a number of scenarios, and strategies for implementing a VPN for each scenario.

2070 * The internal IP addresses of the two networks ***do not collide***. While I expect it is theoretically possible to use a combination of VPN technology and NAT to get this to work, I expect it to be a configuration nightmare.

2074 If you find that you are trying to connect two networks, both of which, internally, use the same private IP address range (e.g., both of them use `192.168.1.x`), then one of the networks will have to be renumbered.

2086 Notice the two public IP addresses. I'll use the letters to refer to them in the rest of this article. Anywhere you see those letters in this article, replace them with your own public IP addresses. Note also that internally, the two gateway machines have .1 IP addresses, and that the two networks have different private IP addresses (`192.168.1.x` and `192.168.2.x` respectively). All the machines on the private networks have been configured to use the `.1` machine as their default gateway.

2090 The intention is that, from a network point of view, each network should view the machines on the other network as though they were directly attached the same router -- albeit a slightly slow router with an occasional tendency to drop packets.

2106 and have it work, transparently. Windows machines should be able to see the machines on the other network, browse file shares, and so on, in exactly the same way that they can browse machines on the local network.

2118 1. Create a ***virtual*** network link between the two networks, across the Internet. Test it, using tools like [ping(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ping&section8), to make sure it works.

2120 1. Apply security policies to ensure that traffic between the two networks is transparently encrypted and decrypted as necessary. Test this, using tools like [tcpdump(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#tcpdump&section1), to ensure that traffic is encrypted.

2130 Suppose that you were logged in to the gateway machine on network #1 (with public IP address `A.B.C.D`, private IP address `192.168.1.1`), and you ran `ping 192.168.2.1`, which is the private address of the machine with IP address `W.X.Y.Z`. What needs to happen in order for this to work?

2136 1. Private IP addresses, such as those in the `192.168.x` range are not supposed to appear on the Internet at large. Instead, each packet you send to `192.168.2.1` will need to be wrapped up inside another packet. This packet will need to appear to be from `A.B.C.D`, and it will have to be sent to `W.X.Y.Z`. This process is called ***encapsulation***.

2142 You can think of this as requiring a ***tunnel*** between the two networks. The two ***tunnel mouths*** are the IP addresses `A.B.C.D` and `W.X.Y.Z`, and the tunnel must be told the addresses of the private IP addresses that will be allowed to pass through it. The tunnel is used to transfer traffic with private IP addresses across the public Internet.

2146 This tunnel is created by using the generic interface, or `gif` devices on DragonFly. As you can imagine, the `gif` interface on each gateway host must be configured with four IP addresses; two for the public IP addresses, and two for the private IP addresses.

2166 Configuring the tunnel is a two step process. First the tunnel must be told what the outside (or public) IP addresses are, using [gifconfig(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#gifconfig&section8). Then the private IP addresses must be configured using [ifconfig(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=ifconfig&section=8).

2266 As the ***Flags*** value indicates, this is a host route, which means that each gateway knows how to reach the other gateway, but they do not know how to reach the rest of their respective networks. That problem will be fixed shortly.

2270 It is likely that you are running a firewall on both machines. This will need to be circumvented for your VPN traffic. You might want to allow all traffic between both networks, or you might want to include firewall rules that protect both ends of the VPN from one another.

2274 It greatly simplifies testing if you configure the firewall to allow all traffic through the VPN. You can always tighten things up later. If you are using [ipfw(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ipfw&section8) on the gateway machines then a command like

2306 However, you will not be able to reach internal machines on either network yet. This is because of the routing -- although the gateway machines know how to reach one another, they do not know how to reach the network behind each one.

2324 This says ***In order to reach the hosts on the network `192.168.2.0`, send the packets to the host `192.168.2.1`***. You will need to run a similar command on the other gateway, but with the `192.168.1.x` addresses instead.

2332 That has now created two thirds of a VPN between the two networks, in as much as it is ***virtual*** and it is a ***network***. It is not private yet. You can test this using [ping(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#ping&section8) and [tcpdump(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=tcpdump&section=1). Log in to the gateway host and run

2380 As you can see, the ICMP messages are going back and forth unencrypted. If you had used the `-s` parameter to [tcpdump(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#tcpdump&section1) to grab more bytes of data from the packets you would see more information.

2438 1. There must be a mechanism for specifying which traffic should be encrypted. Obviously, you don't want to encrypt all your outgoing traffic -- you only want to encrypt the traffic that is part of the VPN. The rules that you put in place to determine what traffic will be encrypted are called ***security policies***.

2442 Security associations and security policies are both maintained by the kernel, and can be modified by userland programs. However, before you can do this you must configure the kernel to support IPsec and the Encapsulated Security Payload (ESP) protocol. This is done by configuring a kernel with:

2462 You have two choices when it comes to setting up security associations. You can configure them by hand between two hosts, which entails choosing the encryption algorithm, encryption keys, and so forth, or you can use daemons that implement the Internet Key Exchange protocol (IKE) to do this for you.

2470 Editing and displaying security policies is carried out using [setkey(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#setkey&section8). By analogy, `setkey` is to the kernel's security policy tables as [route(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=route&section=8) is to the kernel's routing tables. `setkey` can also display the current security associations, and to continue the analogy further, is akin to `netstat -r` in that respect.

2474 There are a number of choices for daemons to manage security associations with DragonFly. This article will describe how to use one of these, racoon. racoon is in the FreeBSD ports collection, in the security/ category, and is installed in the usual way.

2478 racoon must be run on both gateway hosts. On each host it is configured with the IP address of the other end of the VPN, and a secret key (which you choose, and must be the same on both gateways).

2482 The two daemons then contact one another, confirm that they are who they say they are (by using the secret key that you configured). The daemons then generate a new secret key, and use this to encrypt the traffic over the VPN. They periodically change this secret, so that even if an attacker were to crack one of the keys (which is as theoretically close to unfeasible as it gets) it won't do them much good -- by the time they've cracked the key the two daemons have chosen another one.

2486 racoon's configuration is stored in `${PREFIX}/etc/racoon`. You should find a configuration file there, which should not need to be changed too much. The other component of racoon's configuration, which you will need to change, is the ***pre-shared key***.

2490 The default racoon configuration expects to find this in the file `${PREFIX}/etc/racoon/psk.txt`. It is important to note that the pre-shared key is ***not*** the key that will be used to encrypt your traffic across the VPN link, it is simply a token that allows the key management daemons to trust one another.

2494 `psk.txt` contains a line for each remote site you are dealing with. In this example, where there are two sites, each `psk.txt` file will contain one line (because each end of the VPN is only dealing with one other end).

2510 That is, the ***public*** IP address of the remote end, whitespace, and a text string that provides the secret. Obviously, you shouldn't use ***secret*** as your key -- the normal rules for choosing a password apply.

2530 You must run racoon on both gateway machines. You will also need to add some firewall rules to allow the IKE traffic, which is carried over UDP to the ISAKMP (Internet Security Association Key Management Protocol) port. Again, this should be fairly early in your firewall ruleset.

2546 Once racoon is running you can try pinging one gateway host from the other. The connection is still not encrypted, but racoon will then set up the security associations between the two hosts -- this might take a moment, and you may see this as a short delay before the ping commands start responding.

2574 Each IP packet that you send out has a header that contains data about the packet. The header includes the IP addresses of both the source and destination. As we already know, private IP addresses, such as the `192.168.x.y` range are not supposed to appear on the public Internet. Instead, they must first be encapsulated inside another packet. This packet must have the public source and destination IP addresses substituted for the private addresses.

2594 This encapsulation is carried out by the `gif` device. As you can see, the packet now has real IP addresses on the outside, and our original packet has been wrapped up as data inside the packet that will be put out on the Internet.

2610 That's close, but not quite right. If you did this, all traffic to and from `W.X.Y.Z`, even traffic that was not part of the VPN, would be encrypted. That's not quite what you want. The correct policy is as follows

2626 Security policies are also set using [setkey(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#setkey&section8). [setkey(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=setkey&section=8) features a configuration language for defining the policy. You can either enter configuration instructions via stdin, or you can use the `-f` option to specify a filename that contains configuration instructions.

2656 `spdadd` tells [setkey(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#setkey&section8) that we want to add a rule to the secure policy database. The rest of this line specifies which packets will match this policy. `A.B.C.D/32` and `W.X.Y.Z/32` are the IP addresses and netmasks that identify the network or hosts that this policy will apply to. In this case, we want it to apply to traffic between these two hosts. `ipencap` tells the kernel that this policy should only apply to packets that encapsulate other packets. `-P out` says that this policy applies to outgoing packets, and `ipsec` says that the packet will be secured.

2660 The second line specifies how this packet will be encrypted. `esp` is the protocol that will be used, while `tunnel` indicates that the packet will be further encapsulated in an IPsec packet. The repeated use of `A.B.C.D` and `W.X.Y.Z` is used to select the security association to use, and the final `require` mandates that packets must be encrypted if they match this rule.

2726 When they are received by the far end of the VPN they will first be decrypted (using the security associations that have been negotiated by racoon). Then they will enter the `gif` interface, which will unwrap the second layer, until you are left with the innermost packet, which can then travel in to the inner network.

2766 Now, as you can see, [tcpdump(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#tcpdump&section1) shows the ESP packets. If you try to examine them with the `-s` option you will see (apparently) gibberish, because of the encryption.

2790 * Install [`security/racoon`](http://pkgsrc.se/security/racoon). Edit `${PREFIX}/etc/racoon/psk.txt` on both gateway hosts, adding an entry for the remote host's IP address and a secret key that they both know. Make sure this file is mode 0600.

2853 The previous two steps should suffice to get the VPN up and running. Machines on each network will be able to refer to one another using IP addresses, and all traffic across the link will be automatically and securely encrypted.

2869 **OpenSSH** is a set of network connectivity tools used to access remote machines securely. It can be used as a direct replacement for `rlogin`, `rsh`, `rcp`, and `telnet`. Additionally, any other TCP/IP connections can be tunneled/forwarded securely through SSH. **OpenSSH** encrypts all traffic to effectively eliminate eavesdropping, connection hijacking, and other network-level attacks.

2881 Normally, when using [telnet(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#telnet&section1) or [rlogin(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=rlogin&section=1), data is sent over the network in an clear, un-encrypted form. Network sniffers anywhere in between the client and server can steal your user/password information or data transferred in your session. **OpenSSH** offers a variety of authentication and encryption methods to prevent this from happening.

2901 This will load [sshd(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#sshd&section8&manpath=OpenBSD+3.3), the daemon program for **OpenSSH** , the next time your system initializes. Alternatively, you can simply run directly the **sshd** daemon by typing `rcstart sshd` on the command line.

2909 The [ssh(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh&section1&manpath=OpenBSD+3.3) utility works similarly to [rlogin(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=rlogin&section=1).

2929 The login will continue just as it would have if a session was created using `rlogin` or `telnet`. SSH utilizes a key fingerprint system for verifying the authenticity of the server when the client connects. The user is prompted to enter `yes` only when connecting for the first time. Future attempts to login are all verified against the saved fingerprint key. The SSH client will alert you if the saved fingerprint differs from the received fingerprint on future login attempts. The fingerprints are saved in `~/.ssh/known_hosts`, or `~/.ssh/known_hosts2` for SSH v2 fingerprints.

2933 By default, **OpenSSH** servers are configured to accept both SSH v1 and SSH v2 connections. The client, however, can choose between the two. Version 2 is known to be more robust and secure than its predecessor.

2937 The [ssh(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh&section1&manpath=OpenBSD+3.3) command can be forced to use either protocol by passing it the `-1` or `-2` argument for v1 and v2, respectively.

2945 The [scp(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#scp&section1&manpath=OpenBSD+3.3) command works similarly to [rcp(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=rcp&section=1); it copies a file to or from a remote machine, except in a secure fashion.

2965 Since the fingerprint was already saved for this host in the previous example, it is verified when using [scp(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#scp&section1&manpath=OpenBSD+3.3) here.

2969 The arguments passed to [scp(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#scp&section1&manpath=OpenBSD+3.3) are similar to [cp(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=cp&section=1), with the file or files in the first argument, and the destination in the second. Since the file is fetched over the network, through SSH, one or more of the file arguments takes on the form `user@host:<path_to_remote_file>`. The `user@` part is optional. If omitted, it will default to the same username as you are currently logged in as, unless configured otherwise.

2989 Each user can have a personal configuration file in `~/.ssh/config`. The file can configure various client options, and can include host-specific options. With the following configuration file, a user could type `ssh shell` which would be equivalent to `ssh -X user@shell.example.com`.

3045 [ssh-keygen(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh-keygen&section1&manpath=OpenBSD+3.3) will create a public and private key pair for use in authentication. The private key is stored in `~/.ssh/identity`, whereas the public key is stored in `~/.ssh/identity.pub`. The public key must be placed in `~/.ssh/authorized_keys` of the remote machine in order for the setup to work.

3053 **Note:** The `-t rsa1` option will create RSA keys for use by SSH protocol version 1. If you want to use RSA keys with the SSH protocol version 2, you have to use the command `ssh-keygen -t rsa`.

3057 If a passphrase is used in [ssh-keygen(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh-keygen&section1&manpath=OpenBSD+3.3), the user will be prompted for a password each time in order to use the private key.

3061 A SSH protocol version 2 DSA key can be created for the same purpose by using the `ssh-keygen -t dsa` command. This will create a public/private DSA key for use in SSH protocol version 2 sessions only. The public key is stored in `~/.ssh/id_dsa.pub`, while the private key is in `~/.ssh/id_dsa`.

3073 **Warning:** The various options and files can be different according to the **OpenSSH** version you have on your system, to avoid problems you should consult the [ssh-keygen(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh-keygen&section1&manpath=OpenBSD+3.3) manual page.

3125 An SSH tunnel works by creating a listen socket on `localhost` on the specified port. It then forwards any connection received on the local host/port via the SSH connection to the specified remote host and port.

3129 In the example, port `***5023***` on `localhost` is being forwarded to port `***23***` on `localhost` of the remote machine. Since `***23***` is **telnet** , this would create a secure **telnet** session through an SSH tunnel.

3161 This can be used in conjunction with an [ssh-keygen(1)](http://leaf.dragonflybsd.org/cgi/web-man?command#ssh-keygen&section1&manpath=OpenBSD+3.3) and additional user accounts to create a more seamless/hassle-free SSH tunneling environment. Keys can be used in place of typing a password, and the tunnels can be run as a separate user.

3173 At work, there is an SSH server that accepts connections from the outside. On the same office network resides a mail server running a POP3 server. The network, or network path between your home and office may or may not be completely trustable. Because of this, you need to check your e-mail in a secure manner. The solution is to create an SSH connection to your office's SSH server, and tunnel through to the mail server.

3187 When the tunnel is up and running, you can point your mail client to send POP3 requests to `localhost` port 2110. A connection here will be forwarded securely across the tunnel to `mail.example.com`.

3195 Some network administrators impose extremely draconian firewall rules, filtering not only incoming connections, but outgoing connections. You may be only given access to contact remote machines on ports 22 and 80 for SSH and web surfing.

3199 You may wish to access another (perhaps non-work related) service, such as an Ogg Vorbis server to stream music. If this Ogg Vorbis server is streaming on some other port than 22 or 80, you will not be able to access it.